I think the solid base that is a good outline sometimes can be stifling. However, with an outline, one can send out calls for help from co-authors.
In addition, when I'm bored, I can go switch to another part of the project and start outlining.
So for Volume 2, so far I have started pictures, and gotten a handle on chapters that I hadn't started. I also have started Resources and References for the entire project so that's quite useful.
Volumes 3 and 4 both have outlines. More importantly, Volumes 3 and 4 also have my trademark unique chapter names. I've looked at them for Volume 2 but I haven't really had any luck being creative.
It's coming along. I suppose the panic that has been setting in is related to the fact that I will be going back to work in two weeks. Of course the progress would be better if I didn't keep taking naps.